In order to meet the bandwidth increase of optical networks of the future, a rate higher than 100 Gb/s per channel is required. As such, transmission rates of 1 Tb/s and beyond are becoming hot research topics, as described in Y. Ma, Q. Yang, Y. Tang, S. Chen and W. Shieh, “1-Tb/s per channel coherent optical OFDM transmission with subwavelength bandwidth access”, in Proc. OFC, paper PDPC1 (2009); A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, and E. Yoshida, “No-Guard-Interval Coherent Optical OFDM for 100-Gb/s Long-Haul WDM Transmission”, J. Lightw. Technol., vol. 27, no. 16, pp. 3705-3713, 2009; R. Dischler, and F. Buchali, “Transmission of 1.2 Tb/s continuous waveband PDM-OFDM-FDM signal with spectral efficiency of 3.3 bit/S/Hz over 400 km of SSMF”, in Proc. OFC, paper PDPC2 (2009); S. Chandrasekhar et al., “Transmission of a 1.2-Tb/s 24-Carrier no-guard-interval coherent OFDM superchannel over 7200-km of ultra-large-area fiber”, in Proc. ECOC, paper PD 2.6 (2009); J. Yu, X. Zhou, M.-F. Huang, D. Qian, P. N. Ji, T. Wang, and P. Magill, “400 Gb/s (4×100 Gb/s) orthogonal PDM-RZ-QPSK DWDM Signal Transmission over 1040 km SMF-28”, Optics Express, 17, 17928-17933 (2009); D. Hillerkuss et al., “Single source optical OFDM transmitter and Optical FFT receiver demonstrated at line rates of 5.4 and 10.8 Tbit/s”, OFC 2010: PDPC1; and J. Yu, “1.2 Tbit/s orthogonal PDM-RZ-QPSK DWDM signal transmission over 1040 km SMF-28”, Electron. Lett., Vol. 46, No. 11, 2010: 775-777.
Currently, the highest bit rate per channel for optical signal generation is 5.4 Tb/s OFDM polarization multiplexed quadrature phase shift key (PM-QPSK) and 10.8 Tb/s OFDM polarization multiplexed quadrature amplitude modulation with 16 symbol constellation (PM-16QAM) by the comb generation or supercontinuum technique described in D. Hillerkuss et al., “Single source optical OFDM transmitter and Optical FFT receiver demonstrated at line rates of 5.4 and 10.8 Tbit/s”, in Proc. OFC 2010, PDPC1 (2010). However, due to the limited optical signal-to-noise ratio (OSNR) of the OFDM optical signal generation by the supercontinuum technique, transmission distance is quite limited.
Use of a cascaded phase modulator and intensity modulator can generate multi-optical subcarriers as described in J. Yu, X. Zhou, M.-F. Huang, D. Qian, P. N. Ji, T. Wang, and P. Magill, “400 Gb/s (4×100 Gb/s) orthogonal PDM-RZ-QPSK DWDM Signal Transmission over 1040 km SMF-28”, Opt Express., 17, 17928-17933 (2009); J. Yu, “1.2 Tbit/s orthogonal PDM-RZ-QPSK DWDM signal transmission over 1040 km SMF-28”, Electron. Lett, 46, 775-777 (2010); T. Healy et al., “Multi-wavelength source using low drive-voltage amplitude modulators for optical communications,” Opt. Express., 15, 2981-2986 (2007); T. Yamamoto, et al., “Multicarrier light source with flattened spectrum using phase modulators and dispersion medium”, J. of Lightwave. Technol., Vol. 27, No. 19, 2009: 4297-4305; and T. Yamamoto, et al., “multicarrier light source with flattened spectrum using phase modulators and dispersion medium”, J. of Lightwave. Technol., Vol. 27, No. 19, 2009: 4297-4305.
Recently, by using this technique, generation of a 400-Gb/s optical signal has been demonstrated, as described in J. Yu, X. Zhou, M.-F. Huang, D. Qian, P. N. Ji, T. Wang, and P. Magill, “400 Gb/s (4×100 Gb/s) orthogonal PDM-RZ-QPSK DWDM Signal Transmission over 1040 km SMF-28”, Opt Express., 17, 17928-17933 (2009), and generation of a 1.2 Tb/s optical signal is described in J. Yu, “1.2 Tbit/s orthogonal PDM-RZ-QPSK DWDM signal transmission over 1040 km SMF-28”, Electron. Lett, 46, 775-777 (2010).
Due to the limited amplitude of the radio frequency (RF) signals on the phase modulator, only 12 subcarriers (with 25 GHz spacing) are generated, covering an approximate 300 GHz bandwidth with flat spectrum amplitude, as described in J. Yu, “1.2 Tbit/s orthogonal PDM-RZ-QPSK DWDM signal transmission over 1040 km SMF-28”, Electron. Lett, 46, 775-777 (2010). In order to increase the bit rate for the single channel OFDM signals, more subcarriers need to be generated.
A scheme for generating subcarriers using a re-circulating frequency shifter (RFS) has been used to generate 112 subcarriers (or peaks) based upon the frequency shifting a cascade of two phase modulators. However, because the tone-to-noise ratio of the multiple subcarriers generated by RFS is only approximately 20 to 25 dB, it is not enough for high-speed signal transmission over long distance.
A cascade of phase and intensity modulators used to generate multiple subcarriers can achieve a high tone-to-noise ratio with flat amplitude because the subcarriers generated can be used for long distance transmission. However, in order to get flat amplitude, there is a large loss in the intensity modulator (usually over 10 dB loss is needed).